This section provides background information related to the present disclosure which is not necessarily prior art.
Two-photon lithography is a popular technique to additively manufacture (“AM”) complex 3D structures with submicron building blocks (“voxels”). This technique uses a nonlinear photo-absorption process to polymerize submicron features within the interior of the photopolymer resist material. After illumination of the desired structures inside the photoresist volume and subsequent development, the polymerized material remains in the prescribed three-dimensional form.
The availability of well-characterized resists for this process is determined by the ability to measure the mechanical properties of the printed structures. However, this characterization for newly developed custom resists is often hindered by the lack of process knowledge required to successfully fabricate a mechanically stable macroscale part. This deadlock between “print-before-measure” and “measure-before-print” can be resolved via direct metrology on the length scale of the elementary submicron voxel lines. Unfortunately, commercial techniques for such direct measurements on the 100 nm feature scale are not available today.
The specific problem of direct measurement of the mechanical properties of submicron printed features has not been solved in the past. Instead, indirect measurements have been performed by relying on the structural deformation response of assembled printed parts under loading. For example, Bauer et al., “Push-to-pull Tensile Testing of Ultra-strong Nanoscale Ceramic-polymer Composites Made by Additive Manufacturing,” Extreme Mechanics Letters, 2015, have demonstrated indirect measurement of voxel-level properties via a load transfer framework. In addition, Zhang et al., “Controlling Young's Modulus of Polymerized Structures Fabricated by Direct Laser Writing,” Applied Physics A, 118(2), pp. 437-441, 2015, and Cicha et al., “Young's Modulus Measurement of Two-photon Polymerized Micro-cantilevers by Using Nanoindentation Equipment,” Journal of Applied Physics, 112(9), p. 094906, 2012, have demonstrated estimation of average bulk Young's modulus of elasticity by measuring the deformation of assembled structures. All of these techniques presuppose the ability to fabricate a mechanically stable, assembled structure. This is not guaranteed for a newly synthesized custom resist. In addition, all of these techniques generate structure-specific data that cannot be readily generalized beyond the specific structures tested. This is because these techniques comingle the material response (determined by fundamental material properties) and the structural response (determined by structural form), thereby making it infeasible to reliably separate the two effects.
The general problem of direct measurement of the mechanical properties of submicron features has been successfully solved in the past. For example, U.S. Pat. No. 9,279,753 B2 to Espinoza et al. (2016) for “Microelectromechanical device and system”, discloses a microelectromechanical system (“MEMS”) sensor for direct tensile testing of submicron features. In these sensors, the feature of interest is manually transferred to the sensing regions via pick-and-place techniques. The primary limitations of these devices for measurement of printed features are that (a) these sensors cannot be used to incorporate the printed features directly onto the sensors, and (b) pick-and-place techniques cannot be implemented to transfer the printed features onto the sensors. Direct printing of the features onto these sensors is not feasible because of the additional process compatibility requirements imposed by the AM process. Specifically, the liquid-phase development process after the AM step renders the sensors inoperative due to stiction, i.e., due to the effect of moving parts of the sensor collapsing onto each other under the influence of capillary forces generated during development. In addition, pick-and-place techniques for transfer of separately printed features is not practical due to the lower stiffness and strength of printed polymer parts as compared to that of the materials of interest for these prior art sensors (carbon nanotubes, silicon, metals). Thus, existing MEMS sensors for tensile testing are not appropriate for sensing of printed polymer parts.
Accordingly, it would be highly desirable to provide a system capable of directly measuring the mechanical properties of submicron features on a scale that is relevant to additively manufacture larger structures.